The heater of the invention is of the same general type described and claimed in U.S. Pat. No. 4,158,126 which is assigned to the present assignee, and which is used primarily to disinfect soft contact lenses. The heater of the present invention will also be described in conjunction with the disinfecting of soft contact lenses, as well as in conjunction with the disinfecting of dental handpieces and baby bottles although the heater has general applicability, as will become apparent as the description proceeds.
As pointed out in the patent, it is necessary to produce periodically an essentially disinfected condition in soft contact lenses so that bacterial organisms or their by-products will not cause harm to the wearer's eyes. Since the soft lens material is permeable to liquids, soaking the lenses in strong germicidal solution for disinfecting purposes will result in the lenses becoming impregnated with the solution, and this can lead to irritation to the user's eyes when the lenses are worn. In general, it has been found difficult to disinfect soft contact lenses by treatment with chemical or biochemical solutions which will not cause eye irritation to at least some percentage of the wearers.
As an alternate means for producing the desired disinfected condition in the soft contact lenses, and as also pointed out in the patent, heat may be used. The lenses must be kept immersed in psychologically normal saline solution, or its equivalent, when they are not being worn, to prevent the lens material from drying out. Therefore, for disinfecting the lenses they are first placed in a suitable container or lens holder, and a suitable amount of saline solution of proper concentration is added in sufficient quantities so that the lenses are totally immersed. The lense container is then closed and placed in a suitable heating unit for a given time period to disinfect the lenses.
The heating unit must raise the temperature of the saline solution and immersed lenses to the required temperature, and hold the lenses at or above this temperature for the required time, and then allow the lenses to cool back to ambient temperature. Typical values of the time and temperature deemed suitable for disinfecting the lenses require that the lenses be maintained at or above 80.degree. C. for a period of ten minutes or more. Since aging of the lens material is accelerated by excessive temperatures and/or by extended time at elevated temperatures, it is desired that the heating unit be controlled so that excessive temperatures will not shorten the life of the lenses. This is achieved simply and economically in the heater of the present invention.
It is, accordingly, an objective of the present invention to provide an electrically energized heater which is capable of raising the temperature of an item to a desired elevated temperature, and for maintaining the temperature of the item at the elevated temperature for prolonged time periods, without any danger of a critical peak temperature being reached. This desired result is achieved in accordance with the invention by providing a heating unit which is economical in cost, and may be manufactured and sold at a relatively low price.